Therapeutic proteins or peptides in their native state, or when recombinantly produced, are typically labile molecules exhibiting short periods of stability or short serum half-lives. In addition, these molecules are often extremely labile when formulated, particularly when formulated in aqueous solutions for diagnostic and therapeutic purposes. Few practical solutions exist to extend or promote the stability in vivo or in vitro of proteinaceous therapeutic molecules. Many therapeutics, in particular peptide drugs, suffer from inadequate serum half-lives in vivo. This necessitates the administration of such therapeutics at high frequencies and/or higher doses, or the use of sustained release formulations, in order to maintain the serum levels necessary for therapeutic effects. Frequent systemic administration of drugs is associated with considerable negative side effects. For example, frequent, e.g. daily, systemic injections represent a considerable discomfort to the subject, pose a high risk of administration related infections, and may require hospitalization or frequent visits to the hospital, in particular when the therapeutic is to be administered intravenously. Moreover, in long-term treatments daily intravenous injections can also lead to considerable tissue scarring and vascular pathologies caused by the repeated puncturing of vessels. Similar problems are known for all frequent systemic administrations of therapeutics, for example, the administration of insulin to diabetics, or interferon drugs in patients suffering from multiple sclerosis. All these factors lead to a decreased patient compliance and increased costs for the health system.
One possible solution to modify serum half-life of a pharmaceutical agent is to covalently attach to the agent molecules that may increase the half-life. Previously, it has been shown that attachment of polymers, such as polyethylene glycol or “PEG”, to polypeptides may increase their serum half-lives. However, the attachment of polymers can lead to decreases in drug activity. Incomplete or non-uniform attachment leads to a mixed population of compounds having differing properties. Additionally, the changes in half-lives resulting from such modifications are unpredictable. For example, conjugation of different polyethylene glycols to IL-8, G-CSF and IL-1ra produced molecules having a variety of activities and half-lives (Gaertner and Offord, (1996), Bioconjugate Chem. 7:38-44). Conjugation of IL-8 to PEG20 produced no change in its half-life, while conjugation of PEG20 to IL-1ra gave an almost seven-fold increase in half-life. Additionally, the IL-8/PEG20 conjugate was ten- to twenty-fold less effective than the native protein.
Accordingly, methods that are capable of increasing the serum half-life of a biologically active molecule, without seriously diminishing the biological function of the molecule, would be highly desirable.